There are two types of commonly available, gas-fired, warm air furnaces in the marketplace: those with a single gas flow rate, and those with two or more gas flow rates. These are referred to as single and multistage furnaces, respectively. Multistage furnaces are frequently selected by homeowners for replacement furnaces because they offer increased performance and comfort. In retrofit applications there is typically an existing single stage thermostat and wiring in place. It can be troublesome to install a multistage thermostat in a retrofit application when a single stage thermostat is already in place because of the need to route additional wiring through walls for the additional stages. For simple and economical installation, it is desirable to be able to continue to use a single stage thermostat and thermostat wiring when replacing a single stage furnace with a multistage furnace.
Several attempts have been made to allow a single stage thermostat with two-stage furnaces. In some two-stage furnace controls, the controller switches to second stage heating if the demand for heat is not satisfied within a set predetermined time, such as ten minutes. Such furnace controls operate the second stage of heating after some pre-set time has expired, independent of the level of heating actually required at the time.